1. Field of the Invention
This invention relates to an apparatus which controls a trackless moving body adapted to make running movement along a linear guide in the form of an electrically conductive wire or cable so that it can be accurately guided to run along the linear guide.
2. Description of the Prior Art
A transfer crane supported to run with rubber tires or wheels as shown in FIG. 1 is known as an example of a trackless moving body. This transfer crane is used as a cargo-handling machine which hoists and carries a load such as a container in a container yard or terminal.
The crane of this kind includes a trolley 2 capable of making traversing movement on a pair of girders 1, and a spreader 3 vertically movably suspended from the trolley 2 for releasably grasping a container 4. Rubber tires 5, 6, 7 and 8 are mounted on the lower ends of the four legs respectively of the transfer crane shown in FIG. 1. These rubber tires 5 to 8 are arranged at four relative positions as shown in FIG. 2. The rubber tire 5 is driven for rotation by a drive motor 9, and the rubber tire 8 is driven for rotation by another drive motor 10. The remaining rubber tires 6 and 7 are idlers which are merely rotatable relative to the transfer crane since there are no mechanical connections with such drive motors.
The transfer crane is arranged to run along a straight path when the two motors 9 and 10 are simultaneously driven at the same rotation speed. The running direction of the transfer crane can be changed so that it can run leftward or rightward from the straight path when one of the two motors 9 and 10 is deenergized or when the rotation speed of one of the two motors 9 and 10 is made higher or lower than that of the other. The transfer crane can thus be steered to run in any desired direction, and the two motors 9 and 10 serve the dual function of driving and steering the transfer crane.
The transfer crane is driven to run along the straight path under manual control of steering means which controls the operation of the motors 9 and 10. Practically, however, the difference between the diameters of the individual rubber tires 5, 6, 7, 8, the difference between the running resistances and loads imparted to the individual rubber tires 5, 6, 7, 8 and the unbalance of the operating characteristics of the individual motors 9 and 10 make it difficult to run the transfer crane along a straight path in spite of rotation of the two motors 9 and 10 at the same rotation speed, and the running direction of the transfer crane tends to deviate at an angle relative to the straight path. This tendency becomes more marked with the increase in the running speed of the transfer crane. In order to maintain the straightforward running of the transfer crane, a crane operator sitting in an operator's box 11 annexed to the trolley 2 has to effect the necessary steering control. However, the crane operator must also control the traversing movement of the trolley 2 and vertical movement of the spreader 3, and his burden is quite heavy in both the physical aspect and the mental aspect. A skilled crane operator is therefore required for the successful control of the crane operation.
However, it has become more and more difficult to obtain such a skilled operator in recent years, and it has been strongly demanded to furnish the crane of this kind with a function of automatic steering control so that the crane can be automatically guided to run along a straight path or track.
A control apparatus described below is known which is applicable to the crane of the kind above described so that the crane can be automatically controlled to run along a straight path or track.
According to the basic principle of the known control apparatus, a pair of sensors are mounted on the crane in a relation spaced apart from each other in the moving direction of the crane so as to sense the strength of a magnetic field produced by an electrically conductive wire or cable (a linear guide), which will be referred as a conductor cable, buried in the ground surface on which the crane runs, and the sensor output signals are processed in a steering control circuit which generates a steering control signal so that the crane can be guided to run along the conductor cable. The structure of such a steering control circuit is shown in FIG. 3. Referring to FIG. 3, the two sensors generate respective output signals 21 and 22 each indicative of the amount of position deviation from the track and having the polarity varying depending on the direction of position deviation. The signal 21 is applied to an adder 26 and to a differential amplifier 23, while the signal 22 is applied to the differential amplifier 23. In response to the application of the signals 21 and 22, the differential amplifier 23 makes arithmetic calculation to find the difference between the factors of position deviation indicated by the signals 21 and 22, and the difference is then divided by the distance between the two sensors, thereby generating an output signal 25 indicative of the angular deviation of the crane. This angular deviation signal 25 is applied to a differentiation 24 and to the adder 26. The signal 25 applied to the differentiator 24 is differentiated by the differentiator 24, and an output signal 27 indicative of the angular velocity is applied from the differentiator 24 to the adder 26. The adder 26 generates a steering control signal 28 on the basis of the input signals 21, 25 and 27 so that the crane can be guided to run straightforward along the conductor cable or guide wire.
Such a steering control circuit is, however, defective in that it tends to generate a spurious steering control signal 28 because the differentiator 24 itself tends to oscillate. The prior art control circuit is also defective in that the application of the plural signals 21, 25 and 27 to the same adder 26 makes complex and difficult the gain adjustment for the individual signals 21, 25 and 27, and the structure of the circuit becomes inevitably complex too.